Nitroaromatics, particularly dinitrotoluenes (DNT), are widely used as intermediates in the manufacture of aromatic amines, for example toluenediamine, which then can be converted to isocyanates for polyurethane manufacture. Commercially, dinitrotoluenes are typically produced by the mixed acid nitration of toluene, the mixed acid being a mixture of concentrated sulfuric acid and nitric acid. In this process mononitrotoluene is formed in a first nitration stage and then separated from the aqueous phase. The crude mononitrotoluenes are then nitrated with fresh nitrating acid in a second nitration stage. The DNT product is then separated from the aqueous phase for recovery with the aqueous phase being recycled to a nitration stage.
Other methods for producing DNT are taught in U.S. Pat. Nos. 2,362,743; 3,178,481; 3,243,466; and 3,708,546.
U.S. Pat. No. 2,362,743 describes a process for the manufacture of DNT and particularly 2,4-DNT by a two stage nitration process in which no sulfuric acid is used.
U.S. Pat. No. 3,178,481 discloses a method for producing a DNT product having a high percentage of the 2,4-DNT isomer using a loop reaction scheme which makes DNT from toluene in one nitration stage.
U.S. Pat. No. 3,243,466 describes a continuous process for the manufacture of DNT from toluene in a series of four nitration vessels.
U.S. Pat. No. 3,708,546 discloses a process for nitrating toluene to a mixture of DNT isomers high in the 2,4-DNT isomer by reacting the toluene with a mixture of nitric and sulfuric acids at sub-zero temperatures in the presence of a small amount of water. The DNT product is recovered from the reaction mixture by solvent extraction with methylene chloride.
Commercially produced DNT is a combination of six isomers. Each of these isomers has a market of its own in specialty chemicals and the more desirable isomers can be sold at a premium price. The problem, however, is to separate and recover the more desirable isomers from the DNT process and, ideally, not adversely change the composition of the product DNT which must meet certain commercial specifications. Because of this problem, standard separation techniques, for example distillation and crystallization, are usually performed on a "product" grade isomer mixture from a DNT process in order to isolate a particular DNT isomer.
One such separation technique is taught in U.S. Pat. No. 2,765,348 which discloses a process for obtaining 2,6-DNT from its admixture with 2,4-DNT and other impurities. Aniline or a lower alkyl substituted aniline is added to an isomer mixture of 2,4-DNT; 2,6-DNT and impurities. The resulting mixture is cooled below 0.degree. C. but above the temperature at which the mass solidifies. The crystals of 2,6-DNT which form are then recovered.
Nevertheless, there still remains a need for a method for recovering substantially pure 2,4-DNT isomer from a dinitrotoluene process, especially without adversely affecting the isomer makeup of the commercially produced DNT product.